1. Field of the Invention
The present invention relates to improvement technology for improving the display quality in a vertical alignment liquid crystal display.
2. Description of the Related Art
With a vertical alignment liquid crystal display (LCD) in which liquid crystal molecules in a liquid crystal layer disposed between upper and lower substrates are aligned substantially vertically relative to the respective substrates during non-application of voltage, transmittance of a dark state can be lowered extremely by adopting a normally black mode in which the respective polarizers disposed in a manner of sandwiching the upper and lower substrates are of a crossed Nicol arrangement. It is thereby possible to realize a liquid crystal display with a high contrast ratio and superior display quality. A conventional example of this kind of vertical alignment liquid crystal display is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2008-281752.
With the foregoing vertical alignment liquid crystal display, the alignment state of the liquid crystal layer is controlled by applying voltage to the liquid crystal layer by using a lower electrode provided to a lower substrate and an upper electrode provided to an upper substrate. As a method of applying voltage, for instance, the multiplex driving method (simple matrix driving method) is used. As the configuration of a display unit of the liquid crystal display, there are mainly the following two types; namely, a segment type in which electrodes of a shape corresponding to the display image are used, and a dot matrix type in which stripe-shaped electrodes provided respectively to the upper substrate and the lower substrate are arranged so that their respective extending directions intersect, and the areas where the electrodes overlap are respectively used as the pixel. In order to control the liquid crystal layer to be a mono-domain vertical alignment state having a pretilt angle that is slightly inclined relative to the respective substrates during non-application of voltage, a vertical alignment film is provided to the surface of the respective substrates, and the vertical alignment films are subject to an alignment treatment such as the rubbing process. The direction of the alignment treatment is configured, for example, as an anti-parallel state set to the vertical direction (12 o'clock direction, 6 o'clock direction) on the display surface of the liquid crystal display. As the liquid crystal material configuring the liquid crystal layer, a material in which the dielectric constant anisotropy is a negative value is used. When voltage is applied to the liquid crystal layer using the respective electrodes, the liquid crystal molecules are reoriented horizontally relative to the substrate surface along the alignment treatment direction in a large part of the region of the liquid crystal layer. Provided to the outside of the upper and lower substrates are an upper polarizer and a lower polarizer disposed so that their respective absorption axes are substantially orthogonal to each other. The absorption axis of each polarizer is disposed, for example, substantially 45° relative to the alignment treatment direction. A viewing angle compensator is provided between the respective substrates and the respective polarizers as needed. As the viewing angle compensator, used is a compensator having negative uniaxial optical anisotropy or negative biaxial optical anisotropy. When using a viewing angle compensator having negative biaxial optical anisotropy, preferably the in-plane slow axis thereof is substantially orthogonal to the absorption axis of the adjacent polarizer.
With the vertical alignment liquid crystal display described above, the optimal viewing direction with the most superior display quality can be set to the 6 o'clock direction of the display surface. Meanwhile, a direction where this optimal viewing direction is rotated 180° is an anti-viewing direction in which the portion, that is originally of a bright display state when observed at a slight angle from the polar direction based on the normal direction of the display surface, becomes extremely dark and the contrast drops. In the horizontal direction of the display surface, which is a direction that is orthogonal to the optimal viewing direction and the anti-viewing direction, relatively favorable viewing angle characteristics can be obtained. Thus, the liquid crystal display having the foregoing structure can be suitably used for various usages.
Meanwhile, in a conventional liquid crystal display, spherical spacers are distributed and disposed between the upper and lower substrates in order to even out the spacing (cell thickness) between the upper and lower substrates. The cell thickness of the liquid crystal display is defined according to the grain size of the foregoing spherical spacers. However, particularly with a vertical alignment liquid crystal display, regions of alignment non-uniformity tend to arise in the liquid crystal layer around the spherical spacers, and these regions cause a light leak during non-application of voltage and generate a dark region during application of voltage. In order to deal with this drawback, considered may be disposing columnar spacers formed from photosensitive resin between the upper and lower substrates in substitute for the spherical spacers. For example, in the case of a dot matrix-type display unit, by disposing the columnar spacers between the respective stripe-shaped electrodes of the upper and lower substrates, it is possible to improve the display uniformity as well as the cell thickness uniformity.
Nevertheless, even when the columnar spacers are introduced in the vertical alignment liquid crystal display as described above, a sandy texture may be visually recognized across the entire dot matrix-type display unit, for instance, when viewing the liquid crystal display from directions that are roughly 60° in the clockwise and counterclockwise directions, respectively, around the anti-viewing direction (12 o'clock direction). This state where the display non-uniformity is observed most notably near the edge of the pixels and near the edge of the columnar spacers. In addition, near the edge of the columnar spacers, there are cases where the display non-uniformity is also observed from directions other than the anti-viewing direction. Moreover, this kind of display non-uniformity tends to become more pronounced when the frame frequency is lowered in the multiplex driving method.